The invention relates to a radiation detector for detecting low-intensity radiation, in particular for detecting individual photons, and an associated operating method.
The published document Gerhard LUTZ: “Semiconductor radiation detectors”, Springer Verlag, 2nd edition 2001, page 137-152 describes CCD detectors (Charge Coupled Devices) which are used for radiation detection. These known CCD detectors comprise a plurality of parallel image cell rows, each having a plurality of image cells arranged one behind another, in which the radiation to be detected generates signal electrons. These signal electrons are initially held in potential wells which are generated in the region of the individual image cells by an electrode arrangement. By means of suitable electrical control of the electrode arrangement, the signal electrons are further transported along the image cell rows from image cell to image cell and pass within the individual image cell rows to a signal output to which an output amplifier is connected, which amplifies the output signal, so that even low-intensity radiation can be detected. In the case of the known CCD detectors, the output amplifier comprises, for example, a simple transistor which is integrated, together with the CCD detector onto a semiconductor substrate.
However, a disadvantage of the aforementioned known CCD detectors is the unsatisfactory sensitivity, which is insufficient for detecting individual photons with energies of less than 30 eV.
From A. D. HOLLAND: “New developments in CCD and pixel arrays”, Nuclear Instruments and Methods in Physics Research A. vol. 513 (2003), 308-312 and from the publication mentioned there, JERRAM et al.: “The LLLCCD: Low Light Imaging Without the Need for an Intensifier”, Proceedings of SPIE, vol. 4306 (2001), 178-186, a CCD structure with an avalanche amplifier as the output amplifier is known. However, in this prior art, the individual image cell rows of the CCD structure open into a common shift register which shifts the signal electrons serially into a further shift register in which multi-step avalanche amplification takes place.
However, these known detector structures with multi-step serial avalanche amplification have a variety of disadvantages. The serial readout of the signal electrons, for example, leads to a lower read-out speed and to a correspondingly low image refresh rate. Furthermore, the avalanche amplification takes place in multiple steps, which leads to a high level of non-linearity.
Furthermore, from HYNECEK, J.: “CCM—a new low-noise charge carrier multiplier suitable for detection of charge in small pixel CCD image sensors”, IEEE Transactions on Electron Devices, vol. 39, No. 8 (1992), 1972-1975, a CCD detector structure is known wherein avalanche amplification of the signal electrons takes place. However, avalanche amplification takes place within the CCD detector structure and not in a separate output amplifier.
Furthermore, US2005/0083567A1 and WO02/37139A1 describe detector arrangements, although these are less relevant.
It is an object of the invention, in the above described known CCD detector, to improve the sensitivity.
This aim is achieved with a radiation detector according to the invention and a corresponding operating method according to the invention.